Electronic control circuits are used in a wide variety of devices that use electric motors. The control circuit allows a user or system containing the electric motor to adjust various parameters of the power being supplied to the electric motor. As a result, the motor's operation can be controlled. For example, the speed of the electric motor can be adjusted by using the control circuit to limit the amount of current supplied to the motor. Furthermore, the control can be used to vary other aspect of the motor's operation, including but not limited to: rotational velocity, torque and direction of rotation.
Motor control circuits are of importance in barrier operators that are used to actuate access barriers, such as garage doors. However, these electronic motor control circuits are a source of unwanted conducted electromagnetic interference (EMI) emissions which are regulated by the Federal Communications Commission (FCC). Conducted electromagnetic interference (EMI) emissions, are radio frequency (RF) emissions between 150 kHz to 30 MHz which escape an electronic device through the device's electrical power cord. Because EMI emissions are limited by FCC compliance regulations, manufacturers must take affirmative steps to limit the amount of EMI emissions produced by their electronic products as a whole. To achieve EMI emission compliance, manufacturers utilize many techniques.
One method currently used to control a DC motor, while attempting to reduce EMI emissions, involves as a first step, full-wave rectifying the AC power supplied from a standard commercial power source, such as 120 VAC that is input to the motor control. Upon rectification, the AC power is converted into 120V pulsed DC power that is unregulated. To regulate the voltage, the 120V pulsed DC power is “chopped,” using known techniques, so that the average DC voltage supplied to the DC motor can be controlled. Because the electric motor's speed is directly related to the voltage supplied to it, the motor is effectively controlled by adjusting the amount that the 120V pulsed DC power is “chopped.” However, regulating the voltage supplied to the motor by “chopping” results in conducted EMI emissions that typically exceed FCC compliance regulations without appropriate EMI filtering. To overcome this problem, and reduce EMI emissions (as required per FCC regulations), many manufacturers resort to connecting large, bulky EMI filters to the motor control's circuitry. Because of the filter's large size, and the reduced amount of space often allocated for the design of a system or device incorporating the motor control, engineers are not able to achieve their design goals, which are necessary to keep a product's cost and size commensurate with consumer expectations. As a result, engineers must exhaust additional time and resources to develop an alternate design for the system, while sacrificing some of the functionality offered by the original design, in order to meet cost and design constraints. Additionally, the cost of the EMI filters are typically of a heightened expense as compared to the cost of the other components found in the motor control circuit. Furthermore, because the EMI filters add to the overall number of components of the control circuit, there is an elevated likelihood that the control circuit may fail, due to the additional processing and handling required to mount and connect the EMI filter and supporting components on the printed circuit board on which the motor control is mounted.
Therefore, there is a need for a motor control that eliminates or reduces the necessity for the EMI filter components, while still providing reduced EMI emissions. Additionally, there is a need for an electric motor control that controls an electric motor without “chopping” the voltage supplied to the motor, thus reducing EMI emissions. Furthermore, there is a need for an electric motor control that does not need an EMI filter, so that the overall number of components of the motor control can be reduced. Thereby improving the overall reliability of the motor control, while still retaining reduced EMI emissions.